1. The Field of the Invention
This invention relates generally to lightweight concrete block and a method and apparatus for making lightweight concrete block, and more particularly to a method and apparatus for making lightweight concrete blocks that are capable of being dry stacked and have an impact resistant outer surface.
2. Background
The use of lightweight concrete for building construction has been known for decades. Aerated, lightweight concrete has many desirable properties for use in the building construction industry. For example, it is typically easier to handle because of its decreased weight compared to conventional concrete structures. Furthermore, aerated lightweight concrete often has an “R value” or insulative properties that eliminate or substantially decrease the need for additional insulation. Aerated lightweight concrete is also fire resistant so that any buildings built with such materials are less likely to be destroyed by fire.
Aerated, lightweight concrete is typically formed by one of two methods. One method involves mixing cement with an aerated foaming agent to form a cementitious slurry having air cells entrained therein. For example, as disclosed in U.S. Pat. No. 3,062,669 to Dilnot, a lightweight concrete is formed by combining Portland cement, ground silica, fibers, sodium silicate, water and a stable, preformed foam prepared by incorporating air into a hydrolyzed protein foaming agent. Similarly, in U.S. Pat. No. 3,867,159 to Ergene, cellular concrete structures are made by mixing water, cement, and a foam into a foamed cementitious slurry which is then cast in a mold and cured.
Another method includes adding alumina powder to the cement mixture. The alumina powder reacts with the cement mixture to form gas bubbles which cause aeration of the cement mixture. In order to form individual blocks of the lightweight concrete, the aerated cement mixture is often poured into a mold and allowed to harden around the air cells to form an aerated, lightweight concrete block.
Several approaches in the art have been employed to form aerated, lightweight concrete building units that are suitable for building purposes. In order for the lightweight concrete building units to be suitable for building purposes, they must have sufficient structural integrity, e.g., compressive strength, to meet building code requirements and they should be uniform in size and shape to be practical for use in the construction industry. In addition, the building units must be manufacturable in an efficient enough manner and in sufficient quantities to support demands required by the building construction industry. As such, one method known in the art of producing individual blocks is to form larger blocks of lightweight concrete and then cut the larger blocks into smaller building units while the cement is still in a partially cured or “green” state.
When employing cutting methods to form smaller building units, whether the initial larger block is formed by using alumina powder to cause the cement mixture to “rise” or the initial block is formed by forming a foamed cement slurry by adding a stable foam to the mixture, the resultant cement slurry is poured into a large mold and allowed to at least partially cure into a relatively large block. For cement slurries that “rise,’ the height of the block within the mold is dependent upon the amount of aeration or gas generation that occurs within the cement mixture and the amount of gas that is entrained within the cement mixture when the mixture begins to harden. For cement slurries to which a stable foam has been added prior to being poured into a mold, the height of the block in the mold is determined by the amount of prefoamed cement slurry poured into the mold and the amount of air that escapes from the cement slurry before the viscosity of the cement slurry increases to a point where the air cells can no longer migrate within the mixture. Once the cement has hardened or cured to a degree where the formed block can be handled, the block may be removed from the mold and cut into smaller blocks of a desired size and shape. Because the height of the initially formed block is somewhat unpredictable, there is often significant amounts of scrap material produced during such block forming processes. That is, it is often the case that at least a top layer of the initially formed block is wasted. Examples of cutting apparatuses for cutting larger blocks into smaller building units are described in U.S. Pat. No. 4,174,936 to Goransson and U.S. Pat. No. 4,528,883 to Goransson et al.
When casting aerated cement compositions, it is common to find that the density of the block formed varies from top to bottom. That is, prior to solidification of the cement slurry, the gas cells migrate to the top of the block resulting in a block that has a greater density nearer the bottom of the block. Accordingly, individual building units that are cut from a larger block will vary in density resulting in blocks of varying structural strength and weight. In order to compensate for the varying density in individual building units, the density of the entire composition of the aerated slurry must be increased so that the least dense building nits have sufficient structural integrity.
In processes where the individual building units are cut from a larger block, a mortar or some other binding agent must be employed in order to use the building units for construction purposes. In addition, because each building unit has an outer surface that is comprised of open cells, water is easily absorbed into the building units. As such, the surface of the individual building units must typically be treated with a water repellant material to prevent water from absorbing into the block. This is especially important in colder climates where absorption of water can cause the building units to fracture as the water therein expands during solidification. Because of this known phenomenon, such building materials are often required to pass a “freeze/thaw test.” In the freeze/thaw test, the material is submersed in water for an extended period of time and then frozen. If the material cracks, crumbles, or is otherwise structurally compromised, the material will not be approved for use in construction.
Other methods for forming individual building units include individual casting in a mold. For example, as shown in U.S. Pat. No. 5,522,685 to John, the building units are each formed by pouring lightweight cement slurry into a mold to form a cast body and then combining pairs of cast bodies into individual building units. In U.S. Pat. No. 4,372,092 to Lopez, modular panels are individually formed by pouring a cement slurry into a single panel mold having desired components incorporated therein. Such methods for forming individual building units are typically not very efficient at producing large quantities of building units in a relatively short period of time. As such, the cost per unit is relatively high compared to conventional construction materials resulting in products that have not been very commercially successful.
One approach in the art to overcome the foregoing disadvantages with prior art systems is disclosed in U.S. Pat. Nos. 5,457,926 and 5,775,047 to Jensen, the inventor of the present invention, each of which are herein incorporated by this reference. In both references, a lightweight interlocking building block is disclosed in which the blocks may be stacked without the use of mortar. U.S. Pat. No. 5,775,047 teaches that the size of the bubbles entrained in the slurry produce a block having desired compressive and shear strengths. Neither reference, however, describes the method or apparatuses for forming such blocks. Furthermore, there is no teaching of the method or apparatus by which such blocks can be formed in a uniform manner to produce building units having substantially equal dimensions and relatively equivalent densities.
As such it would be advantageous to provide a method and apparatus for forming individual building units in an efficient and cost effective manner.
It would also be advantageous to provide a method and apparatus for forming individual building units having substantially uniform dimensions and relatively consistent densities for each building unit produced.
It would be a further advantage to provide a method and apparatus for forming individual building units in which the building units can be dry stacked without the need for mortar.
It would still be a further advantage to provide a method and apparatus for forming individual building units in which the building units have an impact resistant outer surface.
It would be yet another advantage to provide a method and apparatus for forming individual building units in which the density of the building units can be altered while maintaining the dimensions of the building units.
It would be another advantage to provide a method and apparatus for forming individual building units in which the density of the building units is relatively low while maintaining structural integrity sufficient to meet or exceed building requirements.
It would also be advantageous to provide a method and apparatus for forming individual building units in which the system for forming such building units is fully automated.